Thanos Papanicolaou
IIHR - Hydroscience & Engineering, The University of Iowa

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Application Note: Sediment Transport
 
Cohesive Sediment: Understanding the mechanisms involved in the transport and fate of cohesive sediment in natural channel systems along with effects on aquatic organisms remains an open case in water-related engineering disciplines.  The main challenge is that cohesive sediment dynamics are controlled not only by physical forces (e.g., inertia, buoyancy, drag, lift, friction) but also by electrochemical forces.  A complete identification of the properties of cohesive sediments typically involves twenty-four parameters (Commission of the European Community) and explains why the few studies referring to the transport and fate of cohesive sediment are site-specific rather than have a more fundamental nature. 
   
Channelling:  It has been shown that under certain conditions, fluidization of a mud bed may occur and small volcanoes of fluid may erupt from the mud-water interface.  This process is generally called channelling, which may occur in estuaries and coastal marine environments.  This interesting phenomenon can be described by batch settling tests in the laboratory.  Various theories have been propounded as to the cause of channelling, ranging from specific descriptions of the concentrations at which channelling occurs, to general attributions to “an instability.” A series of laboratory measurements are conducted in order to identify the parameters that govern channelling.  Kaolinite and calcium carbonate powder are used in order to obtain repeatability. A gamma ray scanning system is used to nondestructively obtain density profiles of the settling mixture, over a long period of time.  Sensitive pressure transducers are used to monitor the pore water pressure, and record pressure instabilities which may lead to mechanical parting. A visualization system is also employed, consisting of a high-speed digital camera and fluorescent dye, in order to observe the fluid flow through the consolidating region and to monitor the height of the mud-water interface.  It was concluded that pressure instabilities in the matrix of the consolidating clay lead to the formation of channels and the eruptions of fluid.  This may happen in a manner analogous to the natural hydraulic fracture phenomenon which has been observed by geologists, wherein the poroelastic behavior of rock causes a pressure gradient and corresponding fluid flow. Alternately, the channel formation may be driven by the pore fluid itself, in a manner similar to oil well hydraulic fracture.